Typical of prior art for a wide angle lighting device would be a circular cylindrical fresnel lens in combination with an incandescent lamp as can be found on the buoy lights used to navigate boats.
In this prior art design a cylindrical fresnel or plano-convex lens is formed into a circular pattern about a vertical centerline. This classical buoy light lens is contoured in the vertical plane so that it defines a single focal point located on the vertical centerline. The single focal point is also at the center of the circular pattern formed at the intersection of the horizontal plane and the lens. The incandescent lamp is positioned at the single focal point so that light emerges from the lens with a projected beam pattern that includes a 360 degree beamwidth in the horizontal plane and minimal beamwidth in the vertical plane. This design collects light created by the incandescent source which is emitted at substantial angles above and below the horizontal plane and redirects this light so that it becomes almost parallel to the horizontal plane thus forming an intense beam. Since the incandescent lamp emits light in a substantially uniform spatial radiation pattern the light collected and projected by the lens is substantially uniform in all azimuthal directions of the 360 degree horizontal beam.
A second prior art design also uses the same circular buoy light lens including a single focal point but instead of a single incandescent lamp this configuration incorporates a group of light emitting diode (LED) lamps with lens top bodies. The LED lamps are assembled in a circular formation so that their individual concentrated light beams are directed radially outward from the center of the buoy light lens. The center of the circular formation of LED lamps is coincident with the single focal point of the lens. The single focal point of the buoy light lens works poorly with a plurality of light sources because each of the LED lamps is located at a distance from the single focal point. Since each LED lamp is separated from the focal point, it cannot have its emitted light concentrated into the intense almost parallel beam that could be achieved if it were at the focal point. Generally, the greater the distance between a light source and the focal point the greater the divergence about the horizontal plane of the refracted light emerging from the lighting device. In order to overcome the off-focus location of the LED light sources and achieve acceptably low divergence about the horizontal plane, the body of each LED lamp is contoured to form a lens. The lens on the body of each lamp concentrates the light emitted from the LED element. Although this design uses efficient LED lamps, it is inefficient. Much of the light emitted by the LED element is misdirected within the individual LED lamps due to internal reflection within the bodies of the LED lamps. This internal reflection is related to the light concentrating lens on the body of the LED lamp. Configuring the body of the LED lamp to form a light concentrating lens alters the spatial radiation pattern of the light as it emerges from the body of the lamp. The directional widely divergent spatial radiation pattern of the light emitted from the LED element is altered by the lens so that the light emerging from the LED lamp is directional and concentrated. This alteration is necessary for this prior art design because the buoy light lens cannot--due to the off-focus location of each light source--adequately concentrate the widely divergent light from each LED element. Prior art therefore employs the lens top body of the LED lamp to initiate the concentrating of the light as it leaves the LED lamp body leaving the buoy light lens to complete the concentrating task to finally emit light with minimal divergence about the horizontal plane. Unfortunately, the LED body lens creates several optical problems. Light emerging from the LED body through the body lens and within the concentrated beam pattern appears to the buoy light lens to be emitted from a location different from the location of the LED element. Light emerging from the LED body exterior to the body lens appears to the buoy light lens to be emitted from a multiplicity of points. Thus the light source or LED element appears to the buoy light lens to be larger than its actual size and at multiple locations. It is difficult for any optic to adequately concentrate light emitted from an apparent multiplicity of locations. The buoy light lens of prior art with its single focal point is inadequate for this task.
A third prior art design incorporates a plurality of lens top LED lamps located on the straight horizontal focal line of a straight cylindrical plano-convex lens. Each LED lamp is at the focal point of the lens contour immediately in front of it and light rays emitted by the LED lamp in the vertical plane normal to the lens are refracted to emerge parallel to the horizontal plane. This design is also not efficient because light rays emerging from the LED lamp at azimuthal angles of deviation from the geometric axis of the lamp, intersect the lens to form a contour which defines a focal point at an unacceptably large distance from the LED element. This causes an unacceptable divergence of the light emerging from the plano-convex lens about the horizontal plane. The magnitude of the unacceptable divergence increases as the angle of deviation of the light emerging from the axis of the LED lamp increases. This unacceptable divergence is generally so large that it is difficult to create an emitted light beam of the required concentration or intensity. The lens top body which is included with the LED lamp does help mitigate this problem because it concentrates much of the light emitted by the LED element into a small beam. This reduces the azimuthal divergence of the light emitted from the LED lamp before it impinges upon the plano-convex lens. However the lens top body is counterproductive because it increases the percentage of light lost through Internal reflection within the LED lamp.